1. FIELD OF THE INVENTION
The present invention relates to a mold clamping system, for example for a plastics injection molding machine, in which the clamping system operates to move a movable mold section toward and away from a stationary mold section to define a closed mold cavity to permit high pressure injection molding of a part made from plastics material. More particularly, the present invention relates to a mold clamping system in which the relative movement between the mold sections is performed by a pair of electric motors, each of which drives a respective ball nut to advance a single, non-rotatable screw that is attached to a movable platen that, in turn, carries a movable mold section. The motors selectively move the movable mold section into and out of contact with a stationary mold section mounted on a fixed platen.
2. DESCRIPTION OF THE RELATED ART
Until relatively recently, in commercial practice the mold closing and clamping systems in machines such as die casting or injection molding machines were traditionally hydraulically driven, the hydraulic devices including one or more actuators of the piston-cylinder type for rapid traverse of the movable mold section as well as for applying clamping forces to maintain the mold sections in contact during injection of the material into the mold cavity under high pressure. Further, the use of hydraulic drives extended both to hydraulic-cylinder-operated machines, in which a large diameter movable ram is carried within a large diameter hydraulic cylinder to provide the required high clamping forces to maintain mold sections in closed condition during an injection operation, as well as to toggle-type machines utilizing a series of interconnected links or levers to actuate and control the movement of the movable mold section.
However, with the advent of more sophisticated electrical control systems for monitoring and controlling the various structural elements of molding machines during the various steps involved in a molding cycle, it was found that the use of hydraulic motors and hydraulic actuators resulted in a limitation on the degree of control that could be obtained by such sophisticated electrical control systems. For example, hydraulic-fluid-operated mold clamping systems are not capable of the precision corresponding with the precision attainable with electrical control systems, principally because of the small, yet meaningful, compressibility effects of the hydraulic fluid, as well as the heating and consequent expansion that the hydraulic fluid undergoes during the course of a number of molding cycles. As a result of those factors, an hydraulically-controlled system includes a number of operating factors that each involve a time delay, and consequently the overall cycle time for such systems is greater than that for an electrically-controlled system. In order to reduce cycle time, components capable of greater precision of control and of movement are needed, and an electrical system provides such components.
Although reasonably good control has been achieved in hydraulically-operated clamp systems, as noted above a different drive arrangement must be used if it is desired to attempt to reduce further the overall molding cycle time, which directly influences the cost of the molded parts. Electric motor drives for mold clamping systems had been suggested some years ago (see, for example, U.S. Pat. No. 2,484,712, which issued Oct. 11, 1949, to Conrad Jobst, and United Kingdom Pat. No. 1,136,573, which was published on Dec. 11, 1968), but there has not been widespread use of electric motor drives. Moreover, each of those patents discloses an electrical drive system in which an electric motor drives a mold closing and clamping system through a gear transmission which, because of the accumulation of mechanical tolerances in the several interconnected parts, is incapable of sufficiently precise control of a movable platen position. Furthermore, the above-identified United Kingdom Patent discloses a system utilizing a mechanical flywheel, which results in a system having a great deal of inertia, thereby further rendering difficult the precise and rapid control of the mold closing and clamping system.
More recent patents that disclose electric-motor-operated mold actuation and clamping systems wherein a pair of electric motors is provided for mold movement include U.S. Pat. No. 4,615,669, which issued on Oct. 7, 1986, to Fujita et al.; U.S Pat. No. 4,828,475, which issued on May 9, 1989, to Kamaguchi; U.S. Pat. No. 4,828,476, which issued on May 9, 1989, to Yoshiharu et al.; U.S. Pat. No. 4,877,388, which issued on Oct. 31, 1989, to Inaba et al.; and U.S. Pat. No. 4,929,165, which issued on May 29, 1990, to Inaba et al.
Accordingly, it is an object of the present invention to provide an electric-motor-type drive arrangement for the clamping system of a molding machine.
It is another object of the present invention to provide an electrically-driven mold clamping system in which a pair of motors are provided, one of the motors being operable to effect rapid traverse of a screw connected with a movable platen, while the other motor is operable when the movable platen has reached the limit of its path of travel, in order to provide a high clamping force in order to tightly clamp the movable mold portion against the stationary mold portion.
It is a further object of the present invention to provide an electrically-operated mold clamping system that is capable of precise control to minimize dead time during a molding cycle, and to thereby reduce the overall time for a single operating cycle.